Water-permeable hollow fiber membrane modules have been used commercially for the dehydration of gas streams as well as for the transfer of moisture from one gas stream to another. U.S. Pat. Nos. 6,585,808 and 6,616,735 disclose membrane modules suitable for gas dehydration and are incorporated herein as references and U.S. Pat. No. 6,779,522 discloses a membrane for drying or humidifying gases. For the purpose of describing the salient features of this invention, only bore-side feed gas dehydration modules will be considered, although the membrane module design concepts are applicable to the gas separation and gas transfer applications mentioned previously. In these membrane modules, the feed gas containing the moisture to be removed flows through the lumen of the hollow fiber membranes. As the feed gas flows through the membrane lumen, moisture diffuses across the water permeable membranes to the shell side of the module. In order to maintain this process, a dry gas is injected on the shell side of the module to sweep away the water vapor that has permeated. Often this dry sweep gas is derived from the dry gas that is produced by the membrane module, although other sources may be used.
In an ideal module, not only does the lumen of each fiber receive the same amount of gas flow, but also the sweep gas is uniformly distributed around the outside of the fibers and each fiber is contacted with the same amount of sweep gas. However, in practice some degree of maldistribution of the sweep gas occurs and this results in a loss of performance. To overcome this issue, either additional membrane must be used or additional sweep gas flow must be used, both adding to the cost of the process. Thus, membrane module designers are continually developing new membrane module designs to minimize the maldistribution of the sweep flow and to maximize module performance.
Bikson in U.S. Pat. No. 4,881,955 discloses a membrane module design with improved shell side flow using hollow fibers that are helically wound around a core. However he teaches that the a limitation to this approach is that the fibers must be of essentially uniform length, defining essentially uniform length as “the active lengths of the hollow fibers of the permeator cell will vary from one another by less than about 20 percent” (col 6 lines 41-44).
In addition to performance aspects related to membrane module design, the membrane modules must have the required structural integrity for the applications. In gas drying application, the gas is at elevated pressures and can exceed several hundred psig. This pressure force is applied to the face of the tubesheet and thus the tubesheet must maintain its structural integrity at these forces and at elevated temperatures. In the process of manufacturing a membrane module, the hollow fibers are embedded in a tubesheet. The presence of the fibers in the tubesheet reduces the structural integrity of the tubesheet as the hollow fibers themselves do not add to the strength of the material to resist the pressure forces described above. In addition to the presence of the hollow fibers in the tubesheet, other components may also be present in the tubesheet which tend to reduce the strength of the tubesheet.
For example in Bikson, U.S. Pat. No. 5,026,479 an impervious layer is shown embedded in the tubesheet material. The intersection of the impervious material and the tubesheet results in a discontinuity of the tubesheet material and thus weakens the tubesheet. Thus improvements in the structural integrity of the membrane modules are continually sought.
In Giglia, et al. U.S. Pat. No. 5,837,033 there is shown a hollow fiber membrane module comprising a plurality of helically wound layers of semi-permeable hollow fibers wound on a cylindrical core pipe wherein the fiber wind angle varies across the axial length of the module in one or more layers. In one embodiment, the wind angle of the fibers in the tubesheet region differs (is smaller) than the wind angle in the active region of the module. However, in any embodiment of Giglia, the diameter of the module at the tubesheet is essentially the same as the diameter of the tubesheet. Thus, much of the area of the potting in the tubesheet is occupied by fiber ends. Therefore, the integrity of the tubesheet is compromised by this construction. Additionally, the feed gas pressure drop will be higher in this case due to the greater length of inactive fiber.